Speaking to cheering supporters in Bozeman after his win on Thursday, Mr Gianforte apologised for the incident involving Guardian reporter Ben Jacobs and said he was not proud of his actions.

"I should not have responded the way I did and for that I'm sorry," he said. "I should not have treated that reporter that way."

Mr Gianforte beat Democrat Rob Quist, a banjo player and first-time candidate who had focused his campaign on criticism of the Republican effort to repeal and replace former President Barack Obama's healthcare law.

CNN projected Mr Gianforte would win, and with 96 percent of the vote counted, he led Mr Quist by 51 percent to 43.

He prevailed despite being charged on Wednesday night with assaulting Mr Jacobs, who said the candidate "body-slammed" him during a campaign event in Bozeman.

The assault occurred as Jacobs tried to ask Mr Gianforte about healthcare, according to an audio tape.

Fox News reporter Alicia Acuna, who was preparing to interview Mr Gianforte at the time, said the candidate "grabbed Jacobs by the neck with both hands and slammed him to the ground".

During the scuffle, Jacobs is heard saying: "You just body-slammed me and broke my glasses."

Afterward, three state newspapers rescinded their endorsements of Mr Gianforte.

Some Republican lawmakers, including House Speaker Paul Ryan, suggested he apologise.

Mr Gianforte will take Montana's lone House seat, which was vacated when President Donald Trump named Ryan Zinke as Secretary of the Interior.